The invention relates generally to cryogenic fluid dispensing systems and, more particularly, to a cryogenic liquid fuel dispensing system that utilizes sensor data from a use device receiving the fuel to optimize saturation as the fuel is delivered to a use device fuel tank.
Current alternative fuels include cryogenic substances such as Liquified Natural Gas (LNG). Cryogenic substances have a boiling point generally below xe2x88x92150xc2x0 C. A use device, such as an LNG-powered vehicle, may need to store LNG in an on-board fuel tank with a pressure head that is adequate for the vehicle engine demands. That is, the LNG can be stored in a saturated state on board the vehicle in order to maintain the desired pressure while the vehicle is in motion. This saturation generally occurs by heating the LNG prior to its introduction into the vehicle tank.
LNG is typically dispensed from a bulk storage tank to a vehicle tank by a pressurized transfer. This may be accomplished through the use of a pump, pressurized transfer vessels or a straight pressure transfer from the bulk storage tank at a higher pressure to a vehicle tank at a lower pressure.
A common method of saturating cryogenic liquids, such as LNG, is to saturate the LNG as it is stored in a conditioning tank of a dispensing station. In some instances, the conditioning tank may also be the bulk storage tank of the dispensing station. The LNG may be heated to the desired saturation temperature and pressure by removing LNG from the conditioning tank, warming it, and reintroducing it back into the conditioning tank. The LNG may be warmed, for example, by heat exchangers as illustrated in U.S. Pat. Nos. 5,121,609 and 5,231,838, both to Cieslukowski, and 5,682,750 to Preston et al. Alternatively, the LNG maybe heated to the desired saturation temperature and pressure through the introduction of warmed cryogenic gas into the conditioning tank. Such an approach is illustrated in U.S. Pat. Nos. 5,421,160, 5,421,162 and 5,537,824, all to Gustafson et al.
Saturating the LNG in a dispensing station tank presents a number of disadvantages. One disadvantage is that the vehicle tank may have a higher existing pressure head than is optimum for refueling. If cooler LNG is pumped to the vehicle tank in such situations, the vapor head in the vehicle tank collapses as it encounters the cooler LNG. Such pressure collapse does not occur if saturated LNG is pumped to the vehicle tank, however, and the dispensing station pump may not develop enough pressure to overcome the vehicle tank pressure thereby preventing fuel from flowing to the vehicle. In addition, warming LNG in the dispensing station tank reduces the hold time of the tank. The hold time of the tank is the length of time that the tank may hold the LNG without venting to relieve excessive pressure that builds as the LNG warms. Furthermore, refilling the dispensing tank when it contains saturated LNG requires specialized equipment and takes longer.
While a number of the above difficulties may be overcome by providing an interim dispensing station transfer or conditioning tank, such a system has to be tailored in dimensions and capacities to specific site conditions, that is, the amount of fills, pressures expected, etc. As a result, deviations from the design conditions still results in problems for such a system.
Another approach for saturating the LNG prior to delivery to the vehicle tank is to warm the liquid as it is transferred to the vehicle tank. Such an approach is known in the art as xe2x80x9cSaturation on the Flyxe2x80x9d and is illustrated in U.S. Pat. No. 5,787,940 to Bonn et al. wherein heating elements are provided to heat the LNG as it is dispensed. U.S. Pat. Nos. 5,687,776 to Forgash et al. and 5,771,946 to Kooy et al. also illustrate dispensing systems that use heat exchangers to warm cryogenic liquid fuel as it is transferred to a vehicle. While such prior art xe2x80x9cSaturation on the Flyxe2x80x9d systems remove the difficulties associated with saturating the dispensing station vessel, they do not address issues related to the vehicle tank pressure and temperature since the dispensed LNG fuel enters the vehicle tank at a constant, pre-set temperature.
U.S. Pat. No. 5,373,702 to Kalet et al. presents an LNG delivery system, indicated in general at 50 in FIG. 1, whereby a vehicle fuel tank is initially filled with unheated LNG from a storage tank 52 via lines 54 and 58, pump 56 and coupling 60 to purposely collapse the vapor head therein. The vehicle fuel tank features a spray head positioned in its vapor space through which the LNG from the delivery system flows. The liquid dispensing line 58 includes a pressure sensor 72 which provides an indication to a microprocessor 70 when the liquid level in the vehicle tank reaches the spray head. The microprocessor then manipulates valves 66 and 68 so that LNG is routed through line 62 and a heat exchanger 64. As a result, natural gas vapor is produced and delivered to the vehicle fuel tank so that the LNG therein is saturated. The vehicle includes an overflow tank which receives LNG that is displaced from the vehicle fuel tank as the natural gas vapor is added and saturation occurs. A disadvantage of such an arrangement, however, is the requirement that the vehicle include an overflow tank. This adds to the vehicle cost, weight and complexity. In addition, the pressure sensor 72 only provides an indication of when the back pressure of the flow into the vehicle tank increases, indicating that the vehicle tank is nearly full. As such, pressure sensor 72 does not provide an indication of what the actual pressure within the vehicle tank is.
Accordingly, it is an object of the present invention to provide a cryogenic fuel dispensing system that does not saturate the fuel in a dispensing system tank.
It is another object of the present invention to provide a cryogenic fuel dispensing system whereby fuel may be quickly dispersed at the optimal saturation temperature and pressure.
It is another object of the present invention to maximize the amount of LNG or fluid stored by adding only enough heat to the fluid to achieve the optimal final saturation, thereby creating the maximum possible stored mass of fuel.
It is another object of the present invention to provide a cryogenic fuel dispensing system that initially transfers cooler, unsaturated LNG to a vehicle tank and then saturates the fuel as it is transferred by providing variable levels of heat.
It is still another object of the present invention to provide a cryogenic fuel dispensing system that may reliably refuel vehicles without the need for vehicle-mounted overflow tanks.
It is still another object of the present invention to provide a cryogenic fuel dispensing system that uses sensor data from the vehicle tank to optimize the saturation of the fuel as it is dispensed.
These and other objects will be apparent from the following specification.
The present invention is directed to a system for dispensing cryogenic liquid to a use device tank from a bulk storage tank containing a supply of cryogenic liquid. A dispensing line is in communication with the bulk storage tank and is adapted to communicate with the use device tank. A pump and heater are in circuit with the dispensing line. A system control device, such as a microprocessor, is in communication with the pump and heater so that cryogenic liquid may be dispensed, and selectively heated as it is dispensed, to the use device tank.
A liquid level sensor and a pressure or temperature sensor communicate with the use device tank and the system control device so that the liquid level and temperature or pressure of cryogenic liquid initially in the use device tank may be determined. The system control device uses this information to calculate the amount of heat and cryogenic liquid that must be added to the use device tank to optimally fill the use device tank. The system control device then operates the heater and pump to fill the use device tank with cryogenic liquid saturated as required. Unheated cryogenic liquid is preferably initially added to the use device tank so that the vapor head therein is collapsed. Heat may then be added to the cryogenic liquid stream as it is dispensed prior to the completion of the fill to saturate the liquid and rebuild pressure in the use device tank.
The system may alternatively include only a liquid level sensor in communication with the use device tank. The liquid initially in the use device tank is assumed to be saturated and at the pressure required by the use device when such an embodiment is selected.
The pump is preferably a positive displacement pump and is submerged in cryogenic liquid housed in a sump. The heater may include a heat exchanger, electric heater, cryogenic gas or other heating arrangement.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.